The axion is a hypothetical particle postulated to
solve the Strong CP Problem, i.e. to explain why the
strong interactions conserve the discrete symmetries
P and CP in spite of the fact that these symmetries
are broken in the Standard Model of particle physics
as a whole [1]}, [2]}, [3]}. Axions are independently
motivated by cosmology because a population of cold
axions is produced during the QCD phase transition,
when the temperature is of order 1 GeV (\(k_B = \hbar = c = 1\) ) [4]}. In the simplest scenarios,
axions account for the cold dark matter of the
Universe if the axion mass is of order \(10^{-5}\) 
eV, with large uncertainties [5]}.
